I think you and I are in a similar position, Sue, in that we have one child, and our primary problem now is egg quality. And then we are in a double-bind situation where we need IVF in order to be able to pick the best egg to get over the egg quality problem, but we probably won't produce enough eggs to be able to choose one. So we're forced to carry on naturally, or with superovulation, but the same rubbish eggs keep coming out.
The difference between you and me is that you conceive easily and I don't, and therefore you have evidence of your egg quality; my situation is probably worse in that my eggs won't even fertilise most of the time, but since I have started to conceive this year, I'm now (perhaps unrealistically) hopeful, because I seem to be getting a step nearer. My friend LJ71, who has now given up ttc, had 7 miscarriages, 5 of them on treatment, so this must be down to egg quality. She and I are 42. But she and her second husband have 4 children between them from previous marriages, so it's a little easier to make that decision to stop.
But that's why I'm really (irrationally) hopeful about this treatment I mentioned earlier. If it takes 5 years to develop something that's going to restore fertility, you will still be only 46 when that happens. In my mind it takes the pressure off, if, in theory at least, it's no longer too late.
And like you, I am very scared of having a disabled child, or having to make a terrible decision. If nature hadn't intervened, I would now be 6 months pregnant with a girl with Down's syndrome. That last pregnancy did bring home to me that we were maybe playing with fire at our age.
But despite that personally I think I'm going to always be trying until the menopause intervenes (and then I'm hoping it's reversible!), but that's because I'm not going through miscarriage after miscarriage like you are. I'd almost certainly think differently if I were.